1. Technical Field
This invention relates generally to aircraft gas turbine engines and particularly to turbofan gas turbine engines.
2. Background Art
The operation of turbofan gas turbine aircraft engines is well known. Such engines include a serial arrangement of a fan, a compressor, a combustor and a turbine (the compressor, combustor and turbine comprising a “core engine”). Air admitted into the inlet of the engine is compressed by the engine's compressor. The compressed air is then mixed with fuel in the engine's combustor and burned. The high-energy products of combustion of the burned airfuel mixture then enters the turbine with extracts energy from the mixture in order to drive the compressor and fan. That energy extracted by the turbine above and beyond that necessary to drive the compressor and fan, exits the engine at the core engine exhaust nozzle thereof, producing thrust which powers an associated aircraft. A much larger amount of thrust is produced by the fan which takes in ambient air and accelerates and discharges such air through a fan exhaust nozzle. The ratio of the volumetric flow of air accelerated by the fan to that of the products of combustion discharged from the core exhaust nozzle can be as high as 5-10:1 or even higher.
As aircraft gas turbine engines evolve, they have been required to produce greater and greater quantities of thrust for powering large commercial transport aircraft of ever-increasing capacity, as well as to operate on as little fuel as possible to accommodate the ever-increasing range requirements of such commercial transport aircraft. Recent dramatic escalation in the cost of jet fuel has made the requirements of minimizing the fuel consumption of modern commercial gas turbine aircraft engines even more important.
For efficient operation of such aircraft gas turbine engines, that is, to minimize the amount of fuel required to generate a given amount of thrust, it is necessary that the flow output of both the turbine and fan be precisely controlled as to both speed and direction. Controlling the speed of such flows is achieved in general by controlling the cross sectional flow areas of the core engine and fan exhaust nozzles respectively, by either optimally sizing fixed area nozzles for nominal engine operating conditions or employing variable area exhaust nozzles which can be adjusted in area for optimal flow throughout a range of operating conditions. The geometric shape of the exhaust nozzles themselves controls the direction of flow therethrough.
Both the fan and core engine exhaust nozzles are functionally defined by components of the engine's nacelle. The nacelle includes a core cowl which provides an aerodynamically efficient cover for the core engine extending threrearound and terminating at the downstream end thereof at the engine's exhaust nozzle. The nacelle also includes an outer fan cowl which surrounds the core cowl, enclosing the blades of the fan and defining with the core cowl, an annular fan duct which terminates at the fan exhaust nozzle. Heretofore, the core cowl and fan cowl have been concentric to one another, that is, both such components have shared a common longitudinal center axis such that the fan duct, from the fan inlet to the fan exhaust nozzle is, for the most part, perfectly annular.
The engine and nacelle are typically attached to the underside of the wing of commercial transport airplanes by a pylon which includes a support beam extending generally from a structural member of the aircraft's wing through the nacelle fan cowl and core cowl to the engine's case. Typically this beam is covered by a fairing to aerodynamically smooth the flow around the beam. Thus, it will be appreciated that the pylon must necessarily extend through the fan duct between the fan cowl and core engine cowl. The fairing over the mounting beam somewhat reduces the disturbance to the air flow through the fan duct caused by the pylon, and it has always been felt that the deleterious effect of the pylon's presence in the fan duct was limited to the resistance to the flow through the annular fan duct caused by the pylon.